An aromatic polyimide, which has the highest level of heat resistance among polymers and also has excellent mechanical properties, electric properties, and other properties, is used as a material in various fields.
However, the aromatic polyimide, which typically has poor processability, is unsuitable especially for melt process and as a matrix resin of a fiber-reinforced composite material. To address this, imide oligomers having terminals modified with a thermal cross-linkable group are disclosed. Among them, imide oligomers having terminals modified with 4-(2-phenylethynyl)phthalic anhydride are believed to have excellent balance of moldability, heat resistance, and mechanical properties, and examples of the imide oligomer are disclosed in Patent Documents 1 to 3 and Non-Patent Documents 1 and 2.
Patent Document 1 has an object to provide a highly practical terminal-modified imide oligomer capable of producing a cured product having good heat resistance and mechanical properties and a cured product of the imide oligomer, and discloses a terminal-modified imide oligomer having a logarithmic viscosity number of 0.05 to 1 and obtained by reacting 2,3,3′,4′-biphenyltetracarboxylic dianhydride with a folded and non-planar structure, an aromatic diamine compound, and 4-(2-phenylethynyl)phthalic anhydride and a cured product of the imide oligomer. However, the terminal-modified imide oligomer has a room for improvement in the solubility in an organic solvent and the preservability of a varnish of the imide oligomer.
Patent Document 2 discloses a terminal-modified imide oligomer for infusion and resin transfer molding. In order to impart high flowability to the imide oligomer, a method of reducing the degree of polymerization or of using a diamine having a soft structure is employed. The method improves the solubility of the terminal-modified imide oligomer but is likely to largely lower the glass transition temperature of a terminal cured product.
Patent Document 3 discloses a terminal-modified imide oligomer that is obtained by using a diamine monomer having a sterically bulky structure, but such an imide oligomer typically gives a small elongation at break and is likely to produce a fragile product. Those disclosed in Non-Patent Documents 1 and 2 also achieve insufficient flowability during molding and insufficient physical properties of a cured product after molding.